This invention relates to solar energy collectors having means to avoid the bursting of liquid ways in such collectors caused by freezing of liquid contained therein. Without some form of protection, conventional flat plate solar liquid heaters are subject to damage when exposed to temperatures low enough to cause the heat transfer fluid in the collector to freeze solid. Most already adopted methods for overcoming this problem are not fail-safe. Even if the potential of a collector to freeze is only expected as a rare event, collectors require means to either avoid or protect them against the possibility of freezing. Sub-zero ambient temperatures during periods of no solar radiation can readily result in plate and liquid way temperatures below zero. Even where ambient temperatures fall only as low as 5.degree. C., water filled collectors can freeze due to radiant heat loss to a clear night sky. This effect is observed quite commonly in inland areas at times when low humidity in the air leads to high clarity of the sky, hence high radiant heat loss from a collector.
Freeze protection can be provided by draining the liquid from the collectors, use of antifreezing solutions or warming the liquid in the collectors. These known methods work on the principle of avoiding freezing but have disadvantages. For instance, when an antifreeze solution is used in the collector loop it requires a heat exchanger between the collector and the storage tank. Common antifreeze liquids are ethylene glycol-water and propylene glycol-water solutions. Ethylene glycol is toxic as are some commonly included corrosion inhibitors and most plumbing codes require the use of two metal interfaces between the toxic fluid and the potable water supply. This can be accomplished by use of double walled heat exchangers that may take the form of coils in the water storage tank or external to the tank. Either way entails additional cost. Furthermore, glycols decompose over the extended lifetime expected of solar collectors and require replacement, hence further cost.
Alternatively, air can be used as the heat transfer fluid in the collector heat exchanger loop. Whilst no toxic fluids are involved and no double-walled heat exchanger interface is needed, the disadvantage of air heating collectors is their lower effectiveness than liquid heating collectors.
Another method of freeze protection where potable water is used throughout the tank and collector system, is to pump warm water from the storage tank through the collector when necessary to keep it from freezing. This is disadvantaged by thermal losses from the system being significantly increased and an additional control mode must be provided. In emergencies, when pump power is lost, the collector and piping subject to freezing must be drained, requiring a further control means.
A further method adopted is based solely on draining water from the collectors when they are not operating. Since this usually involves draining heated liquid to waste, it detracts from the effectiveness of the collector system as a whole and adds to running costs. Again, a control mode is required which is not fail-safe.
A fifth method is to design the collector plate and piping so that it will withstand freezing. For example, designs have been proposed using butyl rubber risers and headers that can expand if water freezes in them. Another example of such a passive method of coping with freezing and avoiding bursting of conduits is disclosed in lapsed Australian patent application number 75412/81 of C. D. Doughty. The Doughty invention involves the use of differential heat extracting means associated with each conduit, preferably the extracting means being tapered fins or "flanges" as referred to by Doughty. This way freezing is said to occur progressively from one end of each exposed conduit to the other end and expansion due to freezing occurs out of that other end.
The Doughty apparatus and method suffers a penalty in that the fins disclosed associated with riser type conduits are considerably tapered and, as such, a significant loss of potential flat plate area for solar absorption is lost. Given that it is nearly always desirable to fit a solar collector into an insulated enclosure with a glass or similar transparent layer to enclose the skyward facing surface of the collector, thus to avoid otherwise very significant convective heat loss, it is usually advantageous or necessary to keep the insulated enclosure to a minimum size. The Doughty invention would thus tend to require a significantly increased size collector enclosure because of the loss of usable plate area for absorption of insolation. C. D. Doughty has addressed this problem in a later patent application (GB 2117110A) in which it is said that effective freeze protection can be achieved by associating the tapered flange or fin with both header conduits only in which case it is unnecessary to use other than conventional riser conduits. This assertion is further discussed below. The substantial taper disclosed is indicative of the disadvantage associated with the fact that the heat transfer mechanism of a flat plate or a tapered fin such as described in a glazed enclosure is radiant rather than convective. The purpose of the enclosure is to suppress convection. In actual service as opposed to a normal contrived freeze chamber test situation an enclosed solar collector at risk of freezing is most likely to be in a situation where the glass of the collector box is at a temperature not much below 0.degree. C., the ambient air temperature above the collector being possibly as high as 5.degree. C. The predominant heat transfer by radiation from the fin (or plate) to the collector glazing is by virtue of the temperature difference between the glazing and the plate which is almost certainly extremely small. Thus merely a small amount of fin taper would be insufficient to provide for the mechanism of progressive linear ice formation proposed by Doughty.
In the Doughty specification the effectiveness of the freeze protection disclosed was justified on the basis of a demonstration test in a blast freezing apparatus cooled to -40.degree.. The specification is silent as to whether the test was carried out with the collector panel enclosed in a glazed and insulated enclosure or not. Either way the test might falsely give the impression that the tapered fin construction was more effective than it really would be in practice. On one hand, if not in a collector enclosure, the apparatus would be likely to provide a much greater proportion of convective heat transfer via the fins extending from the liquid conduits. Such a large contribution of convective heat transfer cannot be anticipated in the usual situation where the collector is housed in an insulated glazed enclosure. If, on the other hand, the test were carried out with the apparatus in a collector enclosure, the temperature difference between the glazing and the fins or flanges would be much greater in the described test situation (i.e. about 40.degree. C.) than in practice and so radiant heat transfer from the fins would be given a much larger effect than it would in the normal practical situation. Thus the test situation would increase greatly the likelihood of the tapered fin concept appearing to work as intended, but artificially so. In actual practice the tapered fin idea is too sensitive to spurious small variables to be reliable.
The present invention has been made whilst re-evaluating the concept of total passive protection and the linear progressive freezing of conduits concept as described by Doughty. Also, the present invention seeks to obtain these advantages whilst making use of metallic conduits as distinct from resilient plastics conduits (elsewhere mentioned in other prior art) for the liquid passages in the collector because of the good heat conduction available in metals. The present invention would most likely be advantageous when embodied in liquid conduits of copper, aluminium or corrosion protected steel. However, it could also be applied to collectors made from the majority of commodity plastics materials. These do not possess adequate low temperature resiliency to the extent that they can rely on inherent stretching ability to cope with freezing in a solar collector context.
In common with the Doughty specifications referenced, the present invention would be most likely to be applicable in collectors of the flat plate type having liquid conduits of the header and interconnecting riser type. It would be applicable also to equivalent constructions whether formed by joining tubular members or by inflating straight unbonded passageways between otherwise bonded opposing plates. Such "roll bonded" types feature a single inlet/single outlet joined by branched flow channels, made by roll welding double sheets or either aluminium or copper, and are subjected subsequently to internal pressure to expand the tubes. Any of the types of solar collector referred to in this paragraph would comprise types referred to in this specification and claims as being "of the type described" and would include such collectors when enclosed, in the commonplace manner, in glazed enclosures and insulated below the underside and around the edges of the solar absorbing plate (or finned tubes).
A key matter relates to the method of proving the effectiveness of this invention versus other systems in a simulated test environment. An observation made by the present inventor was that conventional flat plate collectors of the type described filled with potable water and in normal service would fail by bursting of risers even when ambient temperatures fell overnight to 5.degree. C., especially in relatively low populated inland climates where night sky clarity was high. Despite this, identically configured collectors could not be made to fail when simply installed for 3 days in a fan forced freezing chamber in which the air temperature was controlled to -15.degree. C. This led to a suspicion that the test method used by Doughty may not be indicative that the disclosed arrangements would be sufficiently effective in the environments where ice formation in conventional collectors would be very rapid. In the inventor's freeze chamber experiments on collectors of the type described and even on serpentine shaped planar flat plate collectors having a liquid path comprising 10 meters of copper tube in a single length, whilst total freezing of the collector ways could be readily induced, bursting failure could not.
Accordingly, an effective testing arrangement was devised that did enable actual in-service failures to be simulated in the experimental freezing chamber. This enabled the effectiveness of the present invention to be refined and reliably proved in a more expeditious manner than by outdoor field testing. The tests required a radiant heat sink to be installed parallel to and above the collector assembly being tested. The radiant heat sink had a generally planar form and was positioned about 500 mm distant from the collector. The heat sink comprised a 20 m long copper tube formed into a zig-zag or serpentine shape and affixed to a copper plate. The face of the heat sink opposite to the side facing the collector was heat insulated. During testing, a liquid at a temperature of -25.degree. to -30.degree. C. was circulated through the heat sink's copper tube. This arrangement was found to simulate the field test condition of the collector experiencing in effect a black body radiant heat target 5.degree. C. below that of the ambient air temperature and enabled the rapid heat loss and ice formation that caused the busting of the conventional risers in collectors of the type described to be experimentally duplicated.